But how many features then remain? For example, I've seen people discuss font size, version numbers, viewport size, etc. Do the remaining features still make a unique identifier?

In most cases yes. If you upgrade your browser, the only thing that changes is the user agent data. The underlying device remains the same and it is this that leaks a lot of fingerprinting attributes (screen, gfx card, fonts, timezone, language, operating system, battery status, audio setup, bluetooth, installed video codecs, TCP data, IP address if static etc.).

To get a feeling for this, try: https://abrahamjuliot.github.io/creepjs/ ; https://bot.incolumitas.com/ and https://amiunique.org/fingerprint

Combined with super cookies (https://blog.mozilla.org/en/internet-culture/mozilla-explain...), that's a lot of data points to stitch together a high confidence fingerprint.

Although not perfect, FF is much better out of the box at limiting the leaks than chrome.